Maths experts question key ecological theory

Posted on 20 June 2012

Mathematicians at the University of York in the UK and the University of Canterbury in New Zealand say they have disproved a widely accepted theory underpinning the operation of complex networks of interactions in the natural world.

Networks are a powerful way
to describe ecological communities, which typically involve large numbers of
species that can exhibit both negative (eg competition or predation) and
positive (eg mutualism) interactions with one another. Recent mathematical and
computational analysis suggested that nestedness – the
tendency for ecological specialists to interact with a subset of the species
that also interact with more generalist species – increases species richness.

This research will
enable us to better understand the way ecological networks are affected by
environmental fluctuation and climate change

Dr Jon Pitchford

But
the researchers from the York Centre for Complex Systems Analysis (YCCSA) and
the Biomathematics Research Centre at
Canterbury say they have proved the reverse is true, using mathematical models based
on plant-pollinator networks observed in the wild. The data span the globe,
ranging from tropical rainforests to the high Arctic, and include species
such as birds of paradise and hummingbirds as well as insect pollinators such
as bees, wasps and butterflies.

The research is published in
the latest edition of Nature.

By carefully examining
previous analytic results, and applying computational and statistical methods
to 59 empirical datasets representing mutualistic plant-pollinator networks,
they say they disprove the accepted theory of nestedness. Instead, they contend
that the number of mutualistic partners a species has is a much better
predictor of individual species survival and community persistence.

Co-author Dr Jon Pitchford,
who is also a member of the Departments of Biology and Mathematics at York,
said: “We know that real mutualistic communities are nested – they have sets of
interactions-within-interactions, rather like Russian dolls. We are trying to
understand how this is related to their biodiversity and stability. This will
enable us to better understand the way ecological networks are affected by
environmental fluctuation and climate change.”

Co-author
Dr Alex James, of the Department of Mathematics and Statistics at Canterbury,
said: "It is a well-used phrase but correlation does not
imply causation. Although a cursory glance at real networks can make it appear
that nestedness is correlated with survival, you need to delve deeper to
realise this is a secondary correlation. The stronger and more causal
relationship is between the number of mutualistic partners a species has and
its survival."

Co-author Dr Michael Plank,
also of the Department of
Mathematics and Statistics at Canterbury, added: "Real-life
networks, whether they are from ecology, economics, or Facebook, can be large
and complex. This makes it difficult to tease apart causal relationships from
confounding factors. This is where mathematical models come into their own. They
allow us to systematically change one network attribute, such as nestedness,
whilst controlling for other variables."

The researchers were supported by the RSNZ Marsden Fund and Dr Pitchford was supported by the University
of Canterbury Erskine Programme.

Notes to editors:

The
paper ‘Disentangling nestedness from models of
ecological complexity’ is published in that latest issue of Nature. DOI10.1038/nature11214